The present invention relates to lightning protection for fiber composite components. Fiber composite components can be, for example, a part of the fuselage shell or a wing part of an aircraft, a motor vehicle body part, a rotor blade, a mast, or an antenna. In addition, the present invention relates to a method for the automated production of such a fiber composite component having lightning protection.
In the sense of the present description, a fiber composite component is a component that is made up substantially of a fiber composite plastic that substantially contains reinforcing fibers that are embedded in a solid matrix. The reinforcing fibers can preferably be aramid or glass fibers, in particular, carbon fibers or mixtures containing these. The matrix can be a thermosetting or thermoplastic binding agent. The reinforcing fibers are saturated with this binding agent as long as it is liquid. Subsequently, the binding agent cures and fixes the reinforcing fibers.
In the sense of the present description, a laminate construction is to be understood as a plurality of layers made up substantially of reinforcing fibers, the reinforcing fibers being layered one over the other substantially in dry form as strips, bands, textiles, knitted fabrics, braids, and/or strands, but not yet fixedly bound to one another by the cured matrix. A laminate construction is also referred to as a fabric. Here, substantially dry reinforcing fibers are reinforcing fibers that are not impregnated or that have only a slight impregnation, whose function is to prevent slippage of the fibers placed on one another.
In the sense of the present description, a flat metallic structure is a structure made substantially of metal having in one spatial direction a significantly smaller extension than in the orthogonal spatial directions. Such a structure can be a foil, a plate, a mesh, a textile, or a multiplicity of foil strips. According to the present invention, such a flat metallic structure has openings.
In the sense of the present description, openings are preferably to be understood as holes, loops, cavities, and/or through-holes. In the sense of the present description, “impregnating means” refers to a medium or material that adheres to the surface of a substrate, preferably in the form of a liquid, a solidified or viscous liquid, or a powder.
In the sense of the present description, “lightning protection means” refers to a conductive material that can be situated on a surface that is to be protected from lightning strikes and that makes it possible to accept the current that occurs momentarily during a lightning strike in such a way that the surface to be protected suffers no, or only minor, structural damage.
In particular in modern aircraft engineering, components are widely used that are produced with fiber composite plastics. In the structural parts of aircraft, carbon fiber-reinforced thermosetting plastics are preferentially used, but carbon fiber-reinforced thermoplastic high-performance plastics are also sometimes used. In addition, other reinforcing fibers, such as glass fibers or aramid fibers, can also be used. The use of fiber composite plastics provides a significant potential for saving weight, resulting, inter alia, in extended travel range due to reduced fuel consumption. In addition, in comparison with the metallic materials standardly used up to now in aircraft construction, in particular aluminum alloys, plastics have outstanding corrosion resistance and fatigue strength, and as a result in particular servicing and maintenance expenses in the operation of such aircraft can be significantly reduced.
Due to the fact that the electrical conductivity of the fiber composite plastics is, as a rule, significantly lower than that of metallic materials, additional measures must be taken in particular for lightning protection and the return connection to ground of the on-board electrical systems. The return connection to ground takes place for example via additional copper lines having large conductor cross-sections, while the lightning protection is realized by metals integrated into the outer surface of the structural components.
Currently used production methods for structural components, such as partial shells for fuselage sections or wing parts, often make use of the so-called ATL (Automated Tape Laying) method. Here, a so-called pre-preg material, usually in the form of strips, is placed in automated fashion, using a suitable device, on a shape-defining tool in various spatial directions until the fiber composite component to be produced has reached a specified material thickness in all regions. The pre-preg material is reinforcing fiber bundles or strands, in particular carbon fiber strands, pre-impregnated by the producer with a curable thermosetting plastic resin.
After the laying of the pre-preg material, in the known procedures, the lightning protection is applied manually. For this purpose, a strip-shaped copper foil having a large width of, for example, 889 mm (35″) is laid onto the pre-preg material in overlapping strips and is pressed on.
Subsequently, the structure is cured with application of pressure and/or heat, for example using a vacuum bag structure in an autoclave, to form the finished fiber composite component.
This known procedure is characterized by a high degree of manual labor, resulting, inter alia, in high production costs. The strip-by-strip overlapping application of the copper foil achieves good conductivity even transverse to the direction of the strips, and high own weight of the conductive layer.
Another currently used production method for structural components is the automatic placement of dry fibers or strips, also called the DFP (Dry Fiber Placement) method. The fiber or strip material, not pre-impregnated with curable material, is placed in automated fashion, using a suitable device, on the tool in various spatial directions until the fiber composite component to be produced has reached a specified material thickness in all regions. The fixing of the individual fiber strands is accomplished using so-called binders. These are adhesives that can be activated by pressure or temperature. A lightning protection material can be applied automatically, either as a first or as a last layer. Preferably, this takes place in a female-mold process, in which the lightning protection material is placed into a negatively curved mold, for example a concave mold; i.e., the process begins with the placement of the copper layer. The lightning protection material is for example strips of copper mesh that can be produced for example from a copper foil through multiple unidirectional cutting in, and subsequent stretching perpendicular to the run of the cuts, similar to an iron expanded grid. Here, in order to ensure adequate conductivity between the individual copper mesh strips, i.e., transverse to the direction of laying, either further copper mesh strips are applied crosswise, or at least the copper strips significantly overlap one another. The finished dry structure is then infused with a curable thermosetting plastic resin, and, as described above, is cured under application of pressure or vacuum and/or heat to form the finished fiber composite component.
Both the DFP and the ATL method are also referred to as automated fiber placement methods (AFP). Both methods can be carried out in an economically feasible manner only if the laying of the strips or fibers is carried out in automated fashion by a tape or fiber laying machine.
From DE 05819580, it is known to embed a conductive layer, in the form of a mesh or of a foil, in a surface panel.
WO2010/135318 discloses automatic laying of lightning protection material during the production of a composite component.